Prior art surface measurement methods and apparatus are limited to incomplete surface detection due to the effects of shadowing of the light beam by features of the objects being scanned. An example of this is U.S. Pat. No. 4,089,608.
Arrangement of light planes may also create a problem of incorrectly distinguishing light sources for those surfaces that lie beyond the point of intersection of the light planes. U.S. Pat. No. 4,705,401 is an example illustrating this problem.
U.S. Pat. No. 4,737,032 addresses the previously described problems by an arrangement employing a single light source and multiple optical points of view. The multiple points of view are achieved using a single video sensor, and a series of mirrors and optical beam splitting, which increases the complexity of the apparatus.
U.S. Pat. No. 4,705,401 illustrates another problem associated with apparatus that employs light planes to create a contour line on an object. Due to the slope and features of the object as seen from the perspective of the video sensor, the light plane may illuminate very wide areas, or even multiple distinct points on an object surface. When the light plane appears very wide to the sensor, video peak detection methods may detect a video peak which does not coincide with the center of the light plane.
When multiple distinct points on an object surface are illuminated by a singular light plane, multiple video peaks may be detected by the video sensor causing ambiguities and errors in measurement. U.S. Pat. Nos. 4,705,401 and 4,737,032 are both examples which display this problem.
Particularly in linear motion applications, certain arrangements of light sources and video sensor are capable of detecting only the highest surfaces of an object, that is, surfaces closest to the light source, and cannot detect the presence of surface features of the object which may be located physically beneath a higher surface of the object. U.S. Pat. Nos. 4,705,401 and 4,737,032 are both examples that display this limitation. This limitation is caused by the direct association of the X coordinate to the mechanical position of the scanning apparatus.
Due to the direct association between the mechanical X position of the scanning assembly and the object, only one Z measurement can be made for a specific X/Y coordinate. This limitation results in only a 2 dimensional array of Z values being produced, that can describe only the highest surfaces features of an object.